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How to find object like in C++ with findChild(), but from javascript?
As on the QML-side you usually have no access to the parent/child-relationships of the Qt/C++-side, but only to the visual parent/child-relationship, you will need to resort to C++.
You could e.g. create a object, that exposes a method which takes a QObject, and a name and calls the function findChild of this object.
If you only want to find a visual child, you might just implement a recursive bfs over the visual tree in JS and call this.
But as I said in my comment: If you need this, you probably messed up at some other place and should rather think about a way to do it, without findChild(). Using it is not recommended in C++, and is certainly not so in QML.
As it is a recursive search it will do its best in killing your performance. A UI contains easily a thousand elements and you would need to compare the strings all the time. Further you would lie about the dependencies by sneakingly access things you don't tell anyone you are depending on.
The objectName and the object that calls findChild might not be in the same logical part of your code, so it is easily broken, if someone might change this object or the objectName, and you are searching for a name that does not exist anymore.
Additionally, if you found an object with the name - and possibly the right type - there is still no guarantee, it is the right object, as the objectNames are not necessarily unique.
All in all, it is not the best design, to access objects like that - tough it might be possible.
Disclaimer: I have not tried out my proposed solution, as I don't want to waste my time on it.
Reading the wikipedia entry about God Objects, it says that a class is a god object when it knows too much or does too much.
I see the logic behind this, but if it's true, then how do you couple every different class? Don't you always use a master class for connecting window management, DB connections, etc?
The main function/method may know about the existence of the windows, databases, and other objects. It may perform over-arching tasks like introduce the model to the controller.
But that doesn't mean it manages all the little details. It probably doesn't know anything about how the database or windows are implemented.
If it did, it could be accused of being a God object.
A god object is an object that contains references, directly or indirectly, to most if not all objects within an application. As the question observes, it is almost impossible to avoid having a god object in an application. Some object must hold references to the various subsystems: UI, database, communications, business logic, etc. Note that the god object need not be application-defined. Many frameworks have built-in god objects with names like "application context", "application environment", "session", "activator", etc.
The issue is not whether a god object exists, but rather how it is used. I will illustrate with an extreme example...
Let's say that in my application I want to standardize how many decimal places of precision to show when displaying numbers. However, I want the precision to be configurable. I create a class whose responsibility is to convert numbers to strings:
class NumberFormatter {
...
String format(double value) {
int decimalPlaces = getConfiguredPrecision();
return formatDouble(value, decimalPlaces);
}
int getConfiguredPrecision() {
return /* what ??? */;
}
}
The question is, how does getConfiguredPrecision figure out what to return? One way would be to give NumberFormatter a reference to the global application context which it stores in a member field called _appContext. Then we could write:
return _appContext.getPreferenceManager().getNumericPreferences().getDecimalPlaces();
By doing this, we have just made NumberFormatter into a god object as well! Why? Because now we can (indirectly) reference virtually any object in the application through its _appContext field. Is this bad? Yes, it is.
I'm going to write a unit test for NumberFormatter. Let's set up the parameters... it needs an application context?! WTF, that has 57 methods I need to mock. Oh, it only needs the pref manager... WTF, I have to mock 14 methods! Numeric prefs!?! Screw it, the class is simple enough, I don't need to test it...
Let's say that the application context had another method, getDatabaseManager(). Last week we were using SQL, so the method returned an SQL database object. But this week, we've decided to change to a NoSQL database and the method now returns a new type. Is NumberFormatter affected by the change? Hmmm, I can't remember... yeah, it might be, I see it takes an application context in the constructor... let me open the source and take a look... nope, we're in luck: it only accesses getPreferenceManager()... now let's check the other 93 classes that take an application context as a parameter...
This same scenario occurs if a change is made to the preferences manager, or the numeric preferences object. The moral of the story is that an object should only hold references to the things that it needs to perform its job, and only those things. In the case of NumberFormatter, all it needs to know is a single integer -- the number of decimal places. It could be created directly by the application god object who knows the magic number (or the pref manager or better still, numeric prefs), without turning the formatter into a god object itself. Furthermore, any components that need to format numbers could be given a formatter instead of the god object. Wins all around.
So, to summarize, the problem is not the existence of a god object but rather the act of conferring god-like status to other objects willy-nilly.
Incidentally, the design principle that tackles this problem head-on has become known as the Law of Demeter. Or "when paying at a restaurant, give the server your money not your wallet."
In my experience this most often occurs when you're dealing with code that is the product of "Develop as you go" project management (or lack there of). When a project is not thought through and planned and object responsibilities are loose and not delegated properly. In theses scenarios you find a "god-object" being the catchall for code that doesn't have any obvious organization or delegation.
It is not the interconnectedness or coupling of the different classes that is the problem with god-objects, it's the fact that a god-object many times can accomplish most if not all responsibilities of it's derived children, and are fairly unpredictable (by anyone other than the developer) as to what their defined responsibilities are.
Simply knowing about "multiple" classes doesn't make one a God; knowing about multiple classes in order to solve a problem that should be split into several sub-problems does make one a God.
I think the focus should be on whether a problem should be split into several sub-problems, not on the number of classes a given object knows about (as you pointed out, sometimes knowing about several classes is necessary).
Gods are over-hyped.
I'm trying to follow DDD, or a least my limited understanding of it.
I'm having trouble fitting a few things into the DDD boxes though.
An example: I have a User Entity. This user Entity has a reference to a UserPreferencesInfo object - this is just a class which contains a bunch of properties regarding user preferences. These properties are fairly unrelated, other than the fact that they are all user preferences (unlike say an Address VO, where all the properties form a meaningful whole).
Question is - what is this UserPreferencesInfo object?
1) Obviously it's not an Entity (I'm just storing it as 'component' in fluent nhibernate speak (i.e. in the same DB table as the User entity).
2) VO? I understand that Value Object are supposed to be Immutable (so you cant cange them, just new them up). This makes complete sense when the object is an address for instance (the address properties form a meaningful 'whole'). But in the case of UserPreferencesInfo I don't think it makes sense. There could be 100 properties (Realistically) There could be maybe 20 properties on this object - why would I want to discard an recreate the object whenever I needed to change one property?
I feel like I need to break the rules here to get what I need, but I don't really like the idea of that (it's a slippery slope!). Am I missing something here?
Thanks
Answer 1 (the practical one)
I'm a huge proponent of DDD, but don't force it. You've already recognised that immutable VOs add more work than is required. DDD is designed to harness complexity, but in this case there is very little complexity to manage.
I would simply treat UserPreferencesInfo as an Entity, and reference it from the User aggregate. Whether you store it as a Component or in a separate table is your choice.
IMHO, the whole Entity vs VO debate can be rendered moot. It's highly unlikely that in 6 months time, another developer will look at your code and say "WTF! He's not using immutable VOs! What the heck was he thinking!!".
Answer 2 (the DDD purist)
Is UserPreferencesInfo actually part of the business domain? Others have mentioned disecting this object. But if you stick to pure DDD, you might need to determine which preferences belong to which Bounded Context.
This in turn could lead to adding Service Layers, and before you know it, you've over-engineered the solution for a very simple problem...
Here's my two cents. Short answer: UserPreferenceInfo is a value object because it describes the characteristics of an object. It's not an entity because there's no need to track an object instance over time.
Longer answer: an object with 100+ properties which are not related is not very DDD-ish. Try to group related properties together to form new VOs or you might discover new entities as well.
Another DDD smell is to have a lot of set properties in the first place. Try to find the essence of the action instead of only setting the value. Example:
// not ddd
employee.Salary = newSalary;
// more ddd
employee.GiveRaise(newSalary);
On the other hand you may very well have legitimate reasons to have a bunch of properties that are no more than getters and setters. But then there's probably simpler methods than DDD to solve the problem. There's nothing wrong with taking the best patterns and ideas from DDD but relax a little of all the "rules", especially for simpler domains.
I'd say a UserPreferenceInfo is actually a part of the User aggregate root. It should be the responsibility of the UserRepository to persist the User Aggregate Root.
Value objects only need to be newed up (in your object model) when their values are shared. A sample scenario for that would be if you check for a similar UserPreferenceInfo and associate the User with that instead of Inserting a new one everytime. Sharing Value Objects make sense if value object tables would get to large and raise speed/storage concerns. The price for sharing is paid on Insert.
It is reasonable to abstract this procedure in the DAL.
If you are not shraing value objects, there is nothing against updating.
As far as I understand, UserPreferenceInfo is a part of User entity. Ergo User entity is an Aggregate root which is retrieved or saved using UserRepository as a whole, along with UserPreferenceInfo and other objects.
Personally, I think that UserPreferenceInfo is entity type, since it has identity - it can be changed, saved and retrieved from repository and still be regarded as the same object (i.e. has identity). But it depends on your usage of it.
It doesn't matter IMHO how object is represented in the DAL - is it stored in a separate table or part of other table. One of the benefits of DDD is persistence ignorance and is ususally a good thing.
Of course, I may be wrong, I am new to DDD too.
Question is - what is this UserPreferencesInfo object?
I don't know how this case is supported by NHibernate, but some ORMs support special concepts for them. For example DataObjects.Net include Structures concept. It seems that you need something like this in NH.
First time ever posting on a blog. Hope I do it right.
Anyway, since you haven't showed us the UserPreferencesInfo object, I am not sure how it's constructed such that you can have a variable number of things in it.
If it were me, I'd make a single class called UserPreference, with id, userid, key, value, displaytype, and whatever other fields you may need in it. This is an entity. it has an id and is tied to a certain user.
Then in your user entity (the root I am assuming), have an ISet.
100 properties sounds like a lot.
Try breaking UserPreferenceInfo up into smaller (more cohesive) types, which likely/hopefully are manageable as VOs.
I often find myself needing reference to an object that is several objects away, or so it seems. The options I see are passing a reference through a middle-man or just making something available statically. I understand the danger of global scope, but passing a reference through an object that does nothing with it feels ridiculous. I'm okay with a little bit passing around, I suppose. I suspect there's a line to be drawn somewhere.
Does anyone have insight on where to draw this line?
Or a good way to deal with the problem of distributing references amongst dependent objects?
Use the Law of Demeter (with moderation and good taste, not dogmatically). If you're coding a.b.c.d.e, something IS wrong -- you've nailed forevermore the implementation of a to have a b which has a c which... EEP!-) One or at the most two dots is the maximum you should be using. But the alternative is NOT to plump things into globals (and ensure thread-unsafe, buggy, hard-to-maintain code!), it is to have each object "surface" those characteristics it is designed to maintain as part of its interface to clients going forward, instead of just letting poor clients go through such undending chains of nested refs!
This smells of an abstraction that may need some improvement. You seem to be violating the Law of Demeter.
In some cases a global isn't too bad.
Consider, you're probably programming against an operating system's API. That's full of globals, you can probably access a file or the registry, write to the console. Look up a window handle. You can do loads of stuff to access state that is global across the whole computer, or even across the internet... and you don't have to pass a single reference to your class to access it. All this stuff is global if you access the OS's API.
So, when you consider the number of global things that often exist, a global in your own program probably isn't as bad as many people try and make out and scream about.
However, if you want to have very nice OO code that is all unit testable, I suppose you should be writing wrapper classes around any access to globals whether they come from the OS, or are declared yourself to encapsulate them. This means you class that uses this global state can get references to the wrappers, and they could be replaced with fakes.
Hmm, anyway. I'm not quite sure what advice I'm trying to give here, other than say, structuring code is all a balance! And, how to do it for your particular problem depends on your preferences, preferences of people who will use the code, how you're feeling on the day on the academic to pragmatic scale, how big the code base is, how safety critical the system is and how far off the deadline for completion is.
I believe your question is revealing something about your classes. Maybe the responsibilities could be improved ? Maybe moving some code would solve problems ?
Tell, don't ask.
That's how it was explained to me. There is a natural tendency to call classes to obtain some data. Taken too far, asking too much, typically leads to heavy "getter sequences". But there is another way. I must admit it is not easy to find, but improves gradually in a specific code and in the coder's habits.
Class A wants to perform a calculation, and asks B's data. Sometimes, it is appropriate that A tells B to do the job, possibly passing some parameters. This could replace B's "getName()", used by A to check the validity of the name, by an "isValid()" method on B.
"Asking" has been replaced by "telling" (calling a method that executes the computation).
For me, this is the question I ask myself when I find too many getter calls. Gradually, the methods encounter their place in the correct object, and everything gets a bit simpler, I have less getters and less call to them. I have less code, and it provides more semantic, a better alignment with the functional requirement.
Move the data around
There are other cases where I move some data. For example, if a field moves two objects up, the length of the "getter chain" is reduced by two.
I believe nobody can find the correct model at first.
I first think about it (using hand-written diagrams is quick and a big help), then code it, then think again facing the real thing... Then I code the rest, and any smells I feel in the code, I think again...
Split and merge objects
If a method on A needs data from C, with B as a middle man, I can try if A and C would have some in common. Possibly, A or a part of A could become C (possible splitting of A, merging of A and C) ...
However, there are cases where I keep the getters of course.
But it's less likely a long chain will be created.
A long chain will probably get broken by one of the techniques above.
I have three patterns for this:
Pass the necessary reference to the object's constructor -- the reference can then be stored as a data member of the object, and doesn't need to be passed again; this implies that the object's factory has the necessary reference. For example, when I'm creating a DOM, I pass the element name to the DOM node when I construct the DOM node.
Let things remember their parent, and get references to properties via their parent; this implies that the parent or ancestor has the necessary property. For example, when I'm creating a DOM, there are various things which are stored as properties of the top-level DomDocument ancestor, and its child nodes can access those properties via the reference which each one has to its parent.
Put all the different things which are passed around as references into a single class, and then pass around just that one class instance as the only thing that's passed around. For example, there are many properties required to render a DOM (e.g. the GDI graphics handle, the viewport coordinates, callback events, etc.) ... I put all of these things into a single 'Context' instance which is passed as the only parameter to the methods of the DOM nodes to be rendered, and each method can get whichever properties it needs out of that context parameter.
I have a "parent" Obj-C object containing (in a collection) a bunch of objects whose instance variables point to one another, possibly circularly (fear not, no retaining going on between these "siblings"). I write the parent object to XML, which of course involves (among other things) writing out its "children", in no particular order, and due to the possible circularity, I replace these references between the children with unique IDs that each child has.
The problem is reading this XML back in... as I create one "child", I come across an ID, but there's no guarantee the object it refers to has been created yet. Since the references are possibly circular, there isn't even an order in which to read them that solves this problem.
What do I do? My current solution is to replace (in the actual instance variables) the references with strings containing the unique IDs. This is nasty, though, because to use these instance variables, instead of something like [oneObject aSibling] I now have to do something like [theParent childWithID:[oneObject aSiblingID]]. I suppose I could create an aSibling method to simplify things, but it feels like there's a cleaner way than all this. Is there?
This sounds an awful lot like you are re-inventing NSCoding as it handles circular references, etc... Now, there might be a good reason to re-invent that wheel. Only you can answer that question.
In any case, sounds like you want a two pass unarchival process.
Pass 1: Grab all the objects out of the backing store and reconstitute. As each object comes out, shove it in a dictionary or map with the UID as the key. Whenever an object contains a UID, register the object as needing to be fixed up; add it to a set or array that you keep around during unarchival.
Pass 2: Walk the set or array of objects that need to be fixed up and fix 'em up, replacing the UIDs with objects from the map you built in pass #1.
I hit a bit of parse error on that last paragraph. Assuming your classes are sensibly declared, they ought to be able to repair themselves on the fly.
(All things considered, this is exactly the kind of data structure that is much easier to implement in a GC'd environment. If you are targeting Mac OS X, not the iPhone, turning on GC is going to make your life easier, most likely)
Java's serialization process does much the same thing. Every object it writes out, it puts in a 'previously seen objects' table. When it comes to writing out a subsequent reference, if it's seen the object before, then it writes out a code which indicates that it's a previously seen object from the list. When the reverse occurs, whenever it sees such a reference, it replaces it on the fly with the instance before.
That approach means that you don't have to use this map for all instances, but rather the substitution happens only for objects you've seen a second time. However, you still need to be able to uniquely reference the first instance you've got written, whether by some pointer to a part in the data structure or not is dependent on what you're writing.